1. Field of the Invention
The present invention relates to a coordinate input apparatus and, more particularly, to a coordinate input apparatus in which the vibration which was input from a vibration pen is detected by a plurality of sensors attached to a vibration propagating plate and the coordinates of the vibration pen on the vibration propagating plate are detected.
2. Related Background Art
Hitherto, various coordinate input apparatuses using an input pen, a tablet, and the like have been known as apparatuses for inputting hand-written characters, figures, or the like to a processing apparatus such as a computer or the like. In such a kind of system, image data comprising input characters, figures, and the like is output to a display apparatus such as a CRT display or the like or a recording apparatus such as a printer or the like.
The following various kinds of systems have been known in the detection of coordinates on the tablet in such apparatus.
(1) A system in which a change in resistance value of a sheet material arranged so as to face resistance films is detected. PA1 (2) A system in which an electromagnetic or electrostatic induction of conductive sheets or the like arranged so as to face each other is detected. PA1 (3) A system in which an ultrasonic vibration which is propagated from the input pen to the tablet is detected.
However, the above conventional apparatuses have the following drawbacks.
That is, in the case of the type using the resistance film, uniformity of a resistance member directly exerts an influence on the figure inputting accuracy, so that a resistance member having a particularly excellent uniformity is necessary and the costs are relatively high when considering its accuracy and resolution. Since two resistance films, for the X coordinate, and for the Y coordinate are needed, there is also a drawback such that a degree of transparency deteriorates.
On the other hand, in the case of the type using the electromagnetic induction, since electric wires are arranged in a matrix form, the sheet is not transparent. Such a type is improper when the sheet is to be used by overlapping on an original, a display, or the like.
Further, in the case of the conventional coordinate input apparatus using the ultrasonic vibration, there is used a system in which a delay time until the vibration generated by the input pen propagates in the vibration propagating plate and reaches a converting element is detected. In such a system, if the surface wave is used, in the case where a hand is put on the vibration propagating plate or an object is placed thereon while coordinates are being input by the input pen, or in the case where a scratch or the like exists on the surface of the vibration propagating plate, there is a drawback that the surface wave cannot pass through such a portion and the surface wave cannot reach the sensors, so that the coordinates cannot be detected.
On the other hand, the foregoing phenomenon in the case of the plate wave which propagates in a thin plate is not so typical as in the case of the surface wave. Even in the case where a hand is put on the progagating medium, the vibration can pass such a portion although it is slightly attenuated. Therefore, in the case of constructing the coordinate input apparatus according to such a system, the coordinates can be more stably detected by using the plate wave rather than by using the surface wave.
The plate wave is a wave which propagates in a finite thickness and is also called a ram wave. When considering construction, size, and weight of the apparatus and input vibration energy (which largely influences the electric power consumption) and the like, in the case where a material of the vibration propagating plate is, for instance, aluminum, its thickness is set to a value within a range from a fraction of a mm to about 2-3 mm. Therefore, when the coordinate input apparatus increases in size, the vibration propagating plate also increases in size. For instance, when the coordinates are input or the like, the input surface is largely deformed and not only does the operability deteriorate but also the mechanical strength cannot be held for a long time.
On the other hand, in the case of the plate wave elastic wave, not only the vibration occurs in the surface portion of the tablet but also the whole plate itself of the tablet vibrates, so that the back surface also simultaneously vibrates together with the tablet input surface.
Therefore, if the member supporting the tablet to propagate the plate wave elastic wave is made of a material such as a resin or the like which absorbs ultrasonic waves relatively well, the vibration is largely attenuated. Thus, the vibration cannot be stably input by the vibration sensors and the coordinates cannot be detected with a high accuracy. On the other hand, since the effective vibration propagating distance also decreases under the same vibration input conditions, a problem that the size of the input surface is limited also occurs.